U.S. patent application number 14/551113 was filed with the patent office on 2015-08-20 for beam scanning using an interference filter as a turning mirror.
The applicant listed for this patent is APPLE INC.. Invention is credited to Alexander Shpunt.
Application Number | 20150234179 14/551113 |
Document ID | / |
Family ID | 53797991 |
Filed Date | 2015-08-20 |
United States Patent
Application |
20150234179 |
Kind Code |
A1 |
Shpunt; Alexander |
August 20, 2015 |
Beam scanning using an interference filter as a turning mirror
Abstract
Scanning apparatus includes a scanner, which is configured to
scan over a field of view falling within a predefined angular
range. An interference filter is positioned between the scanner and
the field of view and is configured to pass light within a
predefined wavelength range that is incident on the interference
filter at angles within the predefined angular range, while
reflecting the light within the predefined wavelength range that is
incident on the interference filter at an angle that is outside the
predefined angular range. An ancillary optical element communicates
optically with the scanner at a wavelength within the predefined
wavelength range via a beam path that reflects from the
interference filter at the angle that is outside the predefined
angular range.
Inventors: |
Shpunt; Alexander; (Tel
Aviv, IL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
APPLE INC. |
Cupertino |
CA |
US |
|
|
Family ID: |
53797991 |
Appl. No.: |
14/551113 |
Filed: |
November 24, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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61940439 |
Feb 16, 2014 |
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Current U.S.
Class: |
359/212.2 ;
29/592 |
Current CPC
Class: |
Y10T 29/49 20150115;
G02B 26/105 20130101; G02B 26/08 20130101; H04N 1/04 20130101; G02B
5/28 20130101 |
International
Class: |
G02B 26/10 20060101
G02B026/10; G02B 5/28 20060101 G02B005/28; G02B 26/08 20060101
G02B026/08 |
Claims
1. Scanning apparatus, comprising: a scanner, which is configured
to scan over a field of view falling within a predefined angular
range; an interference filter, which is positioned between the
scanner and the field of view and is configured to pass light
within a predefined wavelength range that is incident on the
interference filter at angles within the predefined angular range,
while reflecting the light within the predefined wavelength range
that is incident on the interference filter at an angle that is
outside the predefined angular range; and an ancillary optical
element, which communicates optically with the scanner at a
wavelength within the predefined wavelength range via a beam path
that reflects from the interference filter at the angle that is
outside the predefined angular range.
2. The apparatus according to claim 1, wherein the scanner
comprises a rotating mirror, which directs the beam path over the
predefined angular range as the mirror rotates.
3. The apparatus according to claim 1, wherein the ancillary
optical element comprises a transmitter, which outputs a beam of
light along the beam path toward the interference filter.
4. The apparatus according to claim 3, wherein the predefined
wavelength range contains an emission range of the transmitter.
5. The apparatus according to claim 1, wherein the ancillary
optical element comprises a receiver, which receives a beam of
light along the beam path from the interference filter.
6. The apparatus according to claim 1, wherein the interference
filter comprises a bandpass filter, having a passband that contains
the predefined wavelength range for rays that are incident on the
interference filter at angles within the predefined angular
range.
7. The apparatus according to claim 1, wherein the interference
filter comprises a notch filter, having a stopband that contains
the predefined wavelength range for rays that are incident on the
interference filter at the angle that is outside the predefined
angular range, while allowing the light within the predefined
wavelength range to pass through the interference filter at angles
within the predefined angular range.
8. The apparatus according to claim 1, wherein the interference
filter comprises a high-pass filter, having a band edge at a first
wavelength longer than a maximum wavelength value of the predefined
wavelength range for rays that are incident on the interference
filter at angles within the predefined angular range, wherein for
incidence at the angle that is outside the predefined angular
range, the band edge shifts to a second wavelength that is shorter
than a minimum wavelength value of the predefined wavelength
range.
9. A method for scanning, comprising: operating a scanner to scan
over a field of view falling within a predefined angular range;
positioning between the scanner and the field of view an
interference filter that is configured to pass light within a
predefined wavelength range that is incident on the interference
filter at angles within the predefined angular range, while
reflecting the light within the predefined wavelength range that is
incident on the interference filter at an angle that is outside the
predefined angular range; and directing an ancillary optical
element to communicate optically with the scanner at a wavelength
within the predefined wavelength range via a beam path that
reflects from the interference filter at the angle that is outside
the predefined angular range.
10. The method according to claim 9, wherein operating the scanner
comprises rotating a mirror so as to direct the beam path over the
predefined angular range as the mirror rotates.
11. The method according to claim 9, wherein directing the
ancillary optical element comprises operating a transmitter to
output a beam of light along the beam path toward the interference
filter.
12. The method according to claim 11, wherein the predefined
wavelength range contains an emission range of the transmitter.
13. The method according to claim 9, wherein directing the
ancillary optical element comprises operating a receiver to receive
a beam of light along the beam path from the interference
filter.
14. The method according to claim 9, wherein the interference
filter comprises a bandpass filter, having a passband that contains
the predefined wavelength range for rays that are incident on the
interference filter at angles within the predefined angular
range.
15. The method according to claim 9, wherein the interference
filter comprises a notch filter, having a stopband that contains
the predefined wavelength range for rays that are incident on the
interference filter at the angle that is outside the predefined
angular range, while allowing the light within the predefined
wavelength range to pass through the interference filter at angles
within the predefined angular range.
16. The method according to claim 9, wherein the interference
filter comprises a high-pass filter, having a band edge at a first
wavelength longer than a maximum wavelength value of the predefined
wavelength range for rays that are incident on the interference
filter at angles within the predefined angular range, wherein for
incidence at the angle that is outside the predefined angular
range, the band edge shifts to a second wavelength that is shorter
than a minimum wavelength value of the predefined wavelength
range.
17. A method for producing of an interference filter, the method
comprising: defining an angular range over which a scanner is to
scan over a field of view through the interference filter; defining
a wavelength range of an ancillary optical element for operation in
conjunction with the scanner and an angle outside the defined
angular range at which a beam path between the ancillary optical
element and the scanner is to be incident on the interference
filter; and designing the interference filter so as to pass light
within the defined wavelength range that is incident on the
interference filter at angles within the defined angular range,
while reflecting the light within the defined wavelength range that
is incident on the interference filter at the angle at which the
beam path is to be incident on the interference filter.
18. The method according to claim 17, wherein the ancillary optical
element comprises a beam transmitter, and wherein defining the
wavelength range comprises specifying an emission range of the beam
transmitter.
19. The method according to claim 17, wherein designing the
interference filter comprises choosing an effective refractive
index of the interference filter so as to provide a desired shift
in transmission and reflection of the filter within the defined
wavelength range as a function of the angle of incidence of the
beam path on the filter.
20. The method according to claim 17, wherein the interference
filter is of a type selected from a group of filter types
consisting of a bandpass filter, a notch filter, and a high-pass
filter.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of U.S. Provisional
Patent Application 61/940,439, filed Feb. 16, 2014, which is
incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates generally to methods and
devices for projection and capture of optical radiation, and
particularly to compact optical scanners.
BACKGROUND
[0003] U.S. Patent Application Publication 2013/0207970, whose
disclosure is incorporated herein by reference, describes a
scanning depth engine, which includes a transmitter, which emits a
beam comprising pulses of light, and a scanner, which is configured
to scan the beam, within a predefined scan range, over a scene. A
receiver receives the light reflected from the scene and generates
an output indicative of the time of flight of the pulses to and
from points in the scene. A processor is coupled to control the
scanner and to process the output of the receiver so as to generate
a 3D map of the scene.
[0004] In one of the embodiments disclosed in the above-mentioned
publication, the light from the transmitter reflects off a
beamsplitter and is then directed by a turning mirror (also
referred to as a folding mirror) toward a scanning micromirror.
Light pulses returned from the scene strike the micromirror, which
reflects the light via the turning mirror through the beamsplitter
to the receiver. The beamsplitter may have a bandpass coating, to
prevent light outside the emission band of the transmitter from
reaching the receiver.
SUMMARY
[0005] Embodiments of the present invention provide improved
methods and apparatus for optical scanning.
[0006] There is therefore provided, in accordance with an
embodiment of the present invention, scanning apparatus, which
includes a scanner, which is configured to scan over a field of
view falling within a predefined angular range. An interference
filter is positioned between the scanner and the field of view and
is configured to pass light within a predefined wavelength range
that is incident on the interference filter at angles within the
predefined angular range, while reflecting the light within the
predefined wavelength range that is incident on the interference
filter at an angle that is outside the predefined angular range. An
ancillary optical element communicates optically with the scanner
at a wavelength within the predefined wavelength range via a beam
path that reflects from the interference filter at the angle that
is outside the predefined angular range.
[0007] In a disclosed embodiment, the scanner includes a rotating
mirror, which directs the beam path over the predefined angular
range as the mirror rotates.
[0008] In some embodiments, the ancillary optical element includes
a transmitter, which outputs a beam of light along the beam path
toward the interference filter, wherein the predefined wavelength
range contains an emission range of the transmitter. Additionally
or alternatively, the ancillary optical element includes a
receiver, which receives a beam of light along the beam path from
the interference filter.
[0009] In some embodiments, the interference filter includes a
bandpass filter, having a passband that contains the predefined
wavelength range for rays that are incident on the interference
filter at angles within the predefined angular range.
[0010] In other embodiments, the interference filter includes a
notch filter, having a stopband that contains the predefined
wavelength range for rays that are incident on the interference
filter at the angle that is outside the predefined angular range,
while allowing the light within the predefined wavelength range to
pass through the interference filter at angles within the
predefined angular range.
[0011] In still other embodiments, the interference filter includes
a high-pass filter, having a band edge at a first wavelength longer
than a maximum wavelength value of the predefined wavelength range
for rays that are incident on the interference filter at angles
within the predefined angular range, wherein for incidence at the
angle that is outside the predefined angular range, the band edge
shifts to a second wavelength that is shorter than a minimum
wavelength value of the predefined wavelength range.
[0012] There is also provided, in accordance with an embodiment of
the present invention, a method for scanning, which includes
operating a scanner to scan over a field of view falling within a
predefined angular range. An interference filter is positioned
between the scanner and the field of view. The interference filter
is configured to pass light within a predefined wavelength range
that is incident on the interference filter at angles within the
predefined angular range, while reflecting the light within the
predefined wavelength range that is incident on the interference
filter at an angle that is outside the predefined angular range. An
ancillary optical element is directed to communicate optically with
the scanner at a wavelength within the predefined wavelength range
via a beam path that reflects from the interference filter at the
angle that is outside the predefined angular range.
[0013] There is additionally provided, in accordance with an
embodiment of the present invention, a method for producing of an
interference filter. The method includes defining an angular range
over which a scanner is to scan over a field of view through the
interference filter. A wavelength range of an ancillary optical
element is defined for operation in conjunction with the scanner
and an angle outside the defined angular range at which a beam path
between the ancillary optical element and the scanner is to be
incident on the interference filter. The interference filter is
designed so as to pass light within the defined wavelength range
that is incident on the interference filter at angles within the
defined angular range, while reflecting the light within the
defined wavelength range that is incident on the interference
filter at the angle at which the beam path is to be incident on the
interference filter.
[0014] In a disclosed embodiment, the ancillary optical element
includes a beam transmitter, and defining the wavelength range
includes specifying an emission range of the beam transmitter.
[0015] In some embodiments, designing the interference filter
includes choosing an effective refractive index of the interference
filter so as to provide a desired shift in transmission and
reflection of the filter within the defined wavelength range as a
function of the angle of incidence of the beam path on the
filter.
[0016] The present invention will be more fully understood from the
following detailed description of the embodiments thereof, taken
together with the drawings in which:
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a schematic top view of an optical scanner, in
accordance with an embodiment of the present invention; and
[0018] FIGS. 2A-C, 3A-C, and 4A-C are schematic representations of
idealized filter spectral responses for use in embodiments of the
present invention.
DETAILED DESCRIPTION OF EMBODIMENTS
[0019] The turning mirror in the scanning depth engine of the
above-mentioned U.S. Patent Application Publication 2013/0207970 is
useful in reducing the overall size of the engine. Embodiments of
the present invention achieve a still more compact design and
reduced component count by integrating this sort of beam-turning
function into a bandpass filter element, and thus eliminating
entirely the need for a separate turning mirror. This novel design
is useful not only in scanning depth engines, but also in compact
scanning optical projectors and receivers that may be used in other
applications.
[0020] The disclosed embodiments make use of thin-film interference
filters, which can be engineered to provide blocking and
transmission in given wavelength ranges using techniques of design
and manufacture that are known in the art. The wavelength response
of such an interference filter changes as a function of the angle
of incidence of light rays on the filter, wherein typically the
spectral transmission band of the filter shifts toward shorter
wavelengths as the angle of incidence increases. (The term "light"
is used herein to refer broadly to optical radiation, which may be
in the visible, ultraviolet, or infrared wavelength range.) This
phenomenon of angular filter shift is described, for example, by
Anderson et al., in "Angle-Tuned Thin-Film Interference Filters for
Spectral Imaging," Optics & Photonics News (January, 2011),
pages 12-13, which is incorporated herein by reference. MacLeod
provides further information on this subject in Thin-Film Optical
Filters (Fourth Edition, 2010), and particularly in section 8.4.1,
which is incorporated herein by reference.
[0021] The magnitude of the angular shift of the spectral
transmission of a given filter is controlled by the effective index
of refraction of the filter, n.sub.eff. Typical values of n.sub.eff
are between 1.47 and 2. The lower the value of n.sub.eff, the
greater will be the spectral shift relative to the angle of
incidence. The dependence of the spectral shift of transmission
wavelength .lamda. as a function of angle of incidence .theta. is
expressed by the following formula, given by Anderson et al.:
.lamda. ( .theta. ) = .lamda. ( 0 ) 1 - sin 2 ( .theta. ) n eff 2
##EQU00001##
[0022] Embodiments of the present invention make use of this
feature by applying the same interference coating on a single
substrate to pass light of a given design wavelength when incident
at low angles (i.e., angles near the normal to the substrate),
while reflecting the light when incident at higher angles (farther
from the normal). Thus, the coated substrate can serve both as the
turning mirror for a beam of light that is directed toward it at a
high angle, and as an effectively transparent plate for the same
beam of light when scanned through the filter in a lower range of
angles. As an added benefit, the bandpass filter reduces the
reception of undesired stray light outside the wavelength range of
interest. As noted earlier, this dual use of the coated substrate
facilitates more compact scanner designs with a reduced component
count relative to scanners that are known in the art.
[0023] FIG. 1 is a schematic top view of a scanning engine 20 that
makes use of the angular selectivity of an interference filter 22,
in accordance with an embodiment of the present invention. Filter
22 comprises a suitable substrate, such as a glass plate, with a
coating of multiple thin-film layers that are chosen to give the
desired behavior. A beam transmitter 24, such as a laser, outputs a
beam of light along a beam path 26, which is incident on filter 22
at an angle of incidence .theta..sub.in (measured relative to the
normal of the filter). The beam is reflected from the filter toward
a scanner, such as a rotating mirror 28, which directs the beam
outward through interference filter 22 to scan over a field of view
30 within a predefined angular range. Mirror 28 scans the field of
view over an angular range such that the maximal outgoing angle of
incidence through the filter is .theta..sub.out.sub.--.sub.max.
[0024] As explained above, interference filter 22, which is
positioned between scanning mirror 28 and field of view 30, is
configured to pass light within a predefined wavelength range when
the light is incident on the interference filter at angles within
the predefined angular range of the scanning mirror. At these
angles, filter 22 reflects or otherwise blocks light outside the
predefined wavelength range. Typically, as explained further
hereinbelow, the predefined wavelength range corresponds to the
operating wavelength range of scanning engine 20, which may
correspond, for example, to the emission band of transmitter
24.
[0025] At the same time, filter 22 reflects the light within the
predefined wavelength range that is incident on the interference
filter at a certain angle .theta..sub.in (or practically speaking,
in a range of angles around .theta..sub.in, which can be of
substantial width) outside the predefined angular range. An
ancillary optical element, such as transmitter 24, communicates
optically with the scanner via beam path 26, which reflects from
the interference filter at the angle .theta..sub.in, which is
greater than .theta..sub.out.sub.--.sub.max and is thus outside the
predefined angular range of filter 22.
[0026] Scanning engine 20 may comprise other sorts of ancillary
optical elements, such as a receiver 32, in addition or
alternatively to transmitter 24. For example, in some applications,
such as time-of-flight scanners used in depth mapping, light will
be reflected back from field of view 30 toward filter 22 and
scanning mirror 28 over roughly the range of angles that is defined
by the rotation of the mirror, between 0.degree. and
.theta..sub.out.sub.--.sub.max. Filter 22 will transmit this
incoming beam toward scanning mirror 28, and will then reflect the
beam at a higher angle toward receiver 32 along beam path 26 as
shown in FIG. 1, but in the reverse direction to the beam from
transmitter 24.
[0027] The design parameters of the coating of interference filter
22 are selected so that the wavelength of transmitter 24 falls
within the filter passband for rays that are incident on filter 22
at angles from zero up to .theta..sub.out.sub.--.sub.max. At the
same time, at higher angles, in the vicinity of .theta..sub.in,
filter 22 reflects light at the transmitter wavelength. Typically,
the emission wavelengths of common transmitters, such as
semiconductor lasers, can vary within certain ranges, due to such
factors as production tolerance and temperature, as well as due to
modulation-related band widening. Therefore, filter 22 may be
designed to exhibit the desired angle-dependent behavior over a
range of wavelengths that contains the emission range of
transmitter 24, which extends between minimum and maximum
wavelength values .lamda..sub.L and .lamda..sub.H.
[0028] As illustrated by this embodiment, a major benefit of using
a carefully-designed interference filter 22 in place of a separate
turning mirror is that a given wavelength band (in this case, the
emission band of transmitter 24, tolerances and variations
included) is reflected in a certain angular range and transmitted
in another angular range. The reflection and transmission both take
place through the same physical aperture of the filter, which thus
breaks the inherent geometrical constraints of a conventional
folding mirror. When a conventional folding mirror is used, the
"internal" beam, reflected by the folding mirror prior to
reflection from the scanning mirror, must be separate in space from
the "external" beam reflected from the scanning mirror, for all
orientations of the scanning mirror. Such separation imposes
limitations on the positioning of the folding mirror, which result
in a large physical size of the scanning engine. By contrast, when
interference filter is applied as described herein, no such
physical separation is required, resulting in a much more compact
design.
[0029] FIGS. 2A-C, 3A-C, and 4A-C are schematic representations of
idealized filter spectral responses for use in embodiments of the
present invention. The plots show transmission of filter 22 as
function of wavelength at three different angles of incidence:
normal incidence (.theta.=0.degree.),
.theta..sub.out.sub.--.sub.max, and .theta..sub.in, relative to an
emission range 40 of transmitter 24. The value T=1 corresponds to
full transmission, while T=0 is full reflection. (Of course, actual
filters will exhibit rounded curves, and will not fully reach T=1
or T=0, but designs approximating the responses shown in the
figures can be achieved using techniques that are known in the
art.)
[0030] FIGS. 2A-2C show the response of filter 22 with a narrow
passband 42, while the filter is reflective outside this range. As
explained above and shown in these figures, passband 42 shifts to
shorter wavelength with increasing angle of incidence. For angles
of incidence in the range between 0.degree. and
.theta..sub.out.sub.--.sub.max, as shown respectively in FIGS. 2A
and 2B, the filter passes light of wavelengths in range 40, between
.lamda..sub.L and .lamda..sub.H. At the higher angle
.theta..sub.in, however, the shift of passband 42 to shorter
wavelengths causes filter 22 to reflect wavelengths between
.lamda..sub.L and .lamda..sub.H, as shown in FIG. 2C.
[0031] FIGS. 3A-3C show the response of filter 22 when configured
as a notch filter, with a narrow stopband 52 and passbands
extending above and below the stopband. As in the preceding
embodiment, stopband 52 shifts to shorter wavelength with
increasing angle of incidence. The location and width of stopband
52 are chosen so that filter 22 passes wavelengths in range 40,
between .lamda..sub.L and .lamda..sub.H, for angles in the range
between 0.degree. and .theta..sub.out.sub.--.sub.max, as shown in
FIGS. 3A and 3B. The wavelength shift of stopband 52 at higher
angles, however, causes filter 22 to reflect wavelengths in range
40 for incidence at or near .theta..sub.in, as shown in FIG.
3C.
[0032] FIGS. 4A-4C show the response of filter 22 configured as a
high-pass filter, which passes radiation at wavelengths shorter
than a certain band edge 62 and reflects radiation of longer
wavelengths. Again, band edge 62 shifts to shorter wavelengths with
increasing angle of incidence. The band edge in this case is chosen
so that filter 22 passes wavelengths between .lamda..sub.L and
.lamda..sub.H for angles in the range between 0.degree. and
.theta..sub.out.sub.--.sub.max, as shown in FIGS. 4A and 4B, but
reflects these wavelengths for incidence at or near .theta..sub.in,
as shown in FIG. 4C.
[0033] Referring to the above formula for wavelength shift as a
function of angle, in order to achieve the filter behavior that is
shown in FIGS. 2A-C and 3A-C, the filter design parameters should
be chosen so as to satisfy the relation:
.lamda. H - .lamda. L < .lamda. C [ 1 - ( sin .theta. in n eff )
2 - 1 - ( sin .theta. out _ max n eff ) 2 ] ##EQU00002##
wherein .lamda..sub.C is the center wavelength of the filter. (A
similar formula may be derived to define the band edge behavior of
the high-pass filter illustrated in FIGS. 4A-4C.)
[0034] The above relation is approximate, and the actual filter
behavior will depend on details of the filter design. For example,
practical filter curves will generally deform with angle of
incidence, and not just shift, and polarization splitting may
occur, along with other deviations from ideal behavior. In
practice, the filter layer structure may be optimized to yield
optimal compliance with all requirements, using computer-based
optimization techniques in common use by various vendors.
[0035] Although the formula above can serve as a guideline for
filter design, in practice techniques of filter design, simulation
and fabrication that are known in the art will be used to achieve
the required transmission characteristics for angles in the range 0
to .theta..sub.out.sub.--.sub.max, and reflection for angles around
.theta..sub.in for all laser wavelengths. The value of n.sub.eff
can be chosen, by appropriate choice of filter layer materials and
thicknesses, in order to tune the angular behavior of the filter so
that the filter is reflective for the entire laser wavelength range
at .theta..sub.in and (nearly) fully transmissive for incidence
angles 0.degree. and .theta..sub.out.sub.--.sub.max.
[0036] Although the figures show a certain specific scanner
geometry and filter characteristics, the principles of the present
invention may also be applied, mutatis mutandis, in other scanner
types and using other sorts of filters. For example, it is not
necessary in all embodiments of the present invention that the
transmission range of the interference filter be 0 to
.theta..sub.out.sub.--.sub.maxand reflection range include
.theta..sub.in (with margins of angle tolerances). Rather, it is
sufficient that there be two distinct angular ranges: the
transmission range and the reflection range. Thus, in alternative
embodiments (not shown in the figures), the geometry of the
scanning engine may be modified so that an interference filter
serves as a mirror for received radiation and as transparent cover
glass for the transmitted radiation. Other embodiments that use an
interference filter with disjoint transmit and receive ranges will
be apparent to those skilled in the art and are considered to be
within the scope of the present invention.
[0037] As another example, whereas liquid-crystal-on-silicon (LCOS)
scanners and other types of reflective arrays that are known in the
art use polarized light, with beamsplitters and quarter-wave
plates, to illuminate the array, these elements may be replaced by
an interference filter designed in accordance with the principles
explained above. In this manner, the polarization requirements and
geometrical constraints associate with the array may be
relaxed.
[0038] It will thus be appreciated that the embodiments described
above are cited by way of example, and that the present invention
is not limited to what has been particularly shown and described
hereinabove. Rather, the scope of the present invention includes
both combinations and subcombinations of the various features
described hereinabove, as well as variations and modifications
thereof which would occur to persons skilled in the art upon
reading the foregoing description and which are not disclosed in
the prior art.
* * * * *